Composition of a nickelate composite cathode for a fuel cell

ABSTRACT

In some embodiments, a solid oxide fuel cell comprising an anode, an electrolyte, cathode barrier layer, a nickelate composite cathode separated from the electrolyte by the cathode barrier layer, and a cathode current collector layer is provided. The nickelate composite cathode includes a nickelate compound and second oxide material, which may be an ion conductor. The composite may further comprise a third oxide material. The composite may have the general formula (Ln u M1 v M2 s ) n+1 (Ni 1-t N t ) n O 3n+1 -A 1-x B x O y —C w D z Ce (1-w-z) O 2-δ , wherein A and B may be rare earth metals excluding ceria.

RELATED APPLICATIONS

This Application claims priority to U.S. Prov. Pat. App. No. 62/247,535,filed Oct. 28, 2015, herein incorporated by reference in its entiretyfor all purposes.

This invention was made with Government support under AssistanceAgreement Nos. DE-FE0000303 and DE-FE0012077 awarded by Department ofEnergy. The Government has certain rights in this invention.

TECHNICAL FIELD

The disclosure generally relates to fuel cells, such as solid oxide fuelcells.

BACKGROUND

Fuel cells, fuel cell systems and interconnects for fuel cells and fuelcell systems remain an area of interest. Some existing systems havevarious shortcomings, drawbacks, and disadvantages relative to certainapplications. Accordingly, there remains a need for furthercontributions in this area of technology.

A solid oxide fuel cell may be an electrochemical system configured toconvert fuel (e.g., hydrogen) to electricity at relatively hightemperatures (e.g., greater than about 500 degrees Celsius). In someexamples, lower power degradation rate and lower cost can be achievedwhen operating these systems at lower temperatures. However,polarization of the cathode of the fuel cell may be relatively high atlower temperatures, which can affect system performance.

In some examples, cathodes may be formed of lanthanide nickelate havingthe general formula Ln₂NiO_(4+δ). Lanthanide nickelates may have alayered structure with alternating layers of perovskites and sodiumchloride type layers. The interstitial oxide-ions are accommodated bythe mismatch of the equilibrium (Ln-O) and (M-O) bond lengths where thestructural tolerance factor t is less than 1. This highly mobile O²⁻exhibits a good ionic conductivity. Moreover, in this structure, the Ni(III)/Ni (II) redox couples are pinned at the top of the O²⁻: 2p⁶ bandsto give an acceptably high electronic conductivity in the mixed-valencestate. Due to its unique structure, lanthanide nickelate cathodes mayhave lower activation energy than other cathode materials being used forsolid oxide fuel cells, such as LSM and LSCF. Further, lanthanidenickelate cathode polarization resistance may be less dependent ontemperature change than other materials. Therefore, this material maymaintain lower ASR at lower operating temperatures. Especially low ASRhas been demonstrated from praseodymium nickelate cathode. However, oneissue is that nickelate materials can be unstable under fuel celloperating temperatures, such as between about 700 to about 900 degreesCelsius. For example, under fuel cell operating conditions, thefavorable phase of the nickelate cathode tends to decompose intoundesired phases, which causes fuel cell performance degradation.

Due to its lower ASR, especially at lower temperatures, nickelatecathodes continue to be of interest in the field of fuel cells. In someexamples, A-site doping, such as Sr or Ca, and B-site doping, such asCu, Co, Fe, etc., may be employed in an attempt to stabilize nickelatephase. However, such attempts have achieved limited success and/or otherissues were present, such as higher coefficient of thermal expansion(CTE) of the cathodes, resulting in a mismatch with other fuel cellmaterials or substrate.

Analysis has indicated that nickelate decomposition initiated fromelement exsolution from the A-site of a doped nickelate, such as Prexsolution from Pr₂NiO₄, may result in the formation of oxide. When toomuch A site element exsolutes form nickelate, Ni may become rich on theB-site, and eventually exsolutes from B-site to form NiO. Analysis alsoindicates that exsoluted A-site element tends to diffuse into a cathodeinterlayer made from doped ceria on top of a stabilized zirconiaelectrolyte.

SUMMARY

Example compositions for cathodes of fuels cells, such as, e.g., solidoxide fuels cells, are described. For example, electrochemical fuelcells including cathodes formed of a nickelate composite material aredescribed. The nickelate composite material of the cathode may include anickelate compound and an ionic conductive material. The ionicconductive material may be co-doped ceria. The composition of thenickelate composite cathode may allow for improved long term durabilityand high performance of the cathode and fuel cell under fuel celloperating conditions, for example, as compared to only a nickelatecompound. For example, the nickelate compounds of the nickelatecomposite cathode material may exhibit relatively low area specificresistance (ASR) and better performance, e.g., as compared to othernickelate compounds. The co-doped ceria of the nickelate compositecathode material may be selected to manage material interdiffusionwithin the cathode and increase the phase stability of the nickelate toincrease long term durability of the cathode and fuel cell, e.g., byallowing for exsoluted elements from the A-site and/or B-site of thenickelate compound to diffuse into the ionic phase rather than formingan oxide from the exsolute. In some examples, Ni on the B-site may notdiffuse into the ionic phase (e.g., the Ni on the B-site may beexsoluted and not diffused into the ionic phase). The fuel cell may alsoinclude a cathode barrier layer separating the cathode from anelectrolyte in the fuel cell. The cathode barrier layer may be formed ofa co-doped ceria and may be configured to manage material diffusion(e.g., out of the cathode) and increase cathode phase stability.

In one example, the disclosure relates to a fuel cell comprising ananode; an electrolyte; cathode barrier layer; and a nickelate compositecathode separated from the electrolyte by the cathode barrier layer; anda cathode current collector layer. The nickelate composite cathodeincludes a nickelate compound and an ionic conductive material, and thenickelate compound comprises at least one of Pr₂NiO₄, Nd₂NiO₄,(Pr_(u)Nd_(v))₂NiO₄, (Pr_(u)Nd_(v))₃Ni₂O₇, (Pr_(u)Nd_(v))₄Ni₃O₁₀, or(Pr_(u)Nd_(v)M_(w))₂NiO₄, where M is an alkaline earth metal doped on anA-site of Pr and Nd. The ionic conductive material comprises a firstco-doped ceria with a general formula of (A_(x)B_(y))Ce_(1-x-y)O₂, whereA and B of the first co-doped ceria are rare earth metals. The cathodebarrier layer comprises a second co-doped ceria with a general formula(A_(x)B_(y))Ce_(1-x-y)O₂, where at least one of A or B of the secondco-doped ceria is Pr or Nd. The anode, cathode barrier layer, nickelatecomposite cathode, cathode current collector layer, and electrolyte areconfigured to form an electrochemical cell.

In accordance with some embodiments of the present disclosure, a cathodecomposition comprising nickelate materials and a second oxide material,(A1-xBx)Oy, is provided. IN some embodiments nickelate materials and asecond oxide material (A1-xBx)Oy and a Ceria oxide materialC_(w)D_(z)Ce_((1-w-z))O_(2-δ), which A and B are rare earth elementexcept for Ce element, C and D is rate earth element is provided.

In accordance with some embodiments of the present disclosure, a methodof inhibiting the formation of a lanthanide oxide phase by forming thecathode from a composition comprising a lanthanide nickelate and asecond oxide material which adsorbs an oxide formed from the lanthanideus provided. The second oxide material may comprise rare earth metalsand have a general formula of A(1-x)BxOy, wherein element A and elementB are different rare earth metals excluding cerium. In some embodimentsthe second oxide material may comprise rare earth metals and have ageneral formula of (CwDz)Ce(1-w-z)O2, wherein element C and element Dare different rare earth metals excluding cerium, and the compositionmay further comprise a third oxide material of rare earth metals with ageneral formula of A(1-x)BxOy, wherein element A and element B aredifferent rare earth metals excluding cerium.

In accordance with some embodiments of the present disclosure, a fuelcell is provided. The fuel cell may comprise an anode, an electrolyte, acathode, and a cathode current collector. The cathode may comprise anickelate composite having the general formula(Ln_(u)M1_(v)M2_(s))_(n+1)(Ni_(1-t)N_(t))_(n)O_(3n+1)A_(1-x)B_(x)O_(y)—C_(w)D_(z)Ce_((1-w-z))O_(2-δ)wherein element Ln is a rare earth metal, 0<u≦1, element M1 is a rareearth metal different from element Ln, 0≦v≦1, element M2 is an alkalineearth metal, 0≦s≦0.3, 0.9≦u+v+s<1.1, 1≦n, element N is one or moretransition metals, 0≦t≦0.5, element A is a rare earth metal excludingcerium, element B is a rare earth metal different from element Aexcluding cerium, 0≦x<1, 1.5≦y≦2.0, element C is a rare earth metal,0<w≦0.75, element D is a rare earth metal different from element C,0≦z≦0.75, and 0≦δ≦0.5. The fuel cell may further comprise a cathodebarrier which is disposed between the cathode and the electrolyte,wherein the cathode barrier comprises a co-doped ceria with a generalformula C_(w)D_(z)Ce_((1-w-z))O_(2-δ) wherein element C is a rare earthmetal, 0<w≦0.75, element D is a rare earth metal different from elementC, 0<z≦0.75, and 0≦δ≦0.5.

In accordance with some embodiments of the present disclosure a fuelcell is provided. The fuel cell may comprise an anode, an electrolyte, acathode, and a cathode current collector, wherein said cathode comprisesa nickelate composite having the general formula(Ln_(u)M1_(v)M2_(s))_(n+1)(Ni_(1-t)N_(t))_(n)O_(3n+1)-A_(1-x)B_(x)O_(y)wherein, element Ln is a rare earth metal, 0<u≦1, element M1 is a rareearth metal different from element Ln, 0≦v≦1, element M2 is an alkalineearth metal, 0≦s≦0.3, 0.9≦u+v+s<1.1, 1≦n, element N is one or moretransition metals 0≦t≦0.5, element A is a rare earth metal excludingcerium, element B is a rare earth metal different from element Aexcluding cerium, 0≦x<1, and 1.5≦y≦2.0. The fuel cell may furthercomprise a cathode barrier disposed between the cathode and theelectrolyte, wherein said cathode barrier comprises a co-doped ceriawith a general formula C_(w)D_(z)Ce_(1-w-z))O_(2-δ) wherein, element Cis a rare earth metal, 0<w≦0.75, element D is a rare earth metaldifferent from element C, 0<z≦0.75, and 0≦δ≦0.5.

In accordance with some embodiments of the present disclosure, a fuelcell having a nickelate cathode is provided. The fuel cell may comprisean anode, an electrolyte, a cathode, and a cathode current collector.The cathode may comprise a nickelate composite having the generalformula(Ln_(u)M1_(v)M2_(s))_(n+1)(Ni_(1-t)N_(t))_(n)O_(n+1)—C_(w)D_(z)Ce_((1-w-z))O_(2-δ),wherein element Ln is a rare earth metal, 0<u≦1, element M1 is a rareearth metal different from element Ln, 0≦v≦1, element M2 is an alkalineearth metal, 0≦s≦0.3, 0.9≦u+v+s<1.1, 1≦n, element N is one or moretransition metals, 0≦t≦0.5, element C is a rare earth metal, 0<w≦0.75,element D is a rare earth metal different from element C, 0<z≦0.75, and0≦δ≦0.5. The fuel cell may further comprise a cathode barrier disposedbetween the cathode and the electrolyte.

The details of one or more embodiments of the disclosure are set forthin the accompanying drawings and the description below. Other features,objects, and advantages of the disclosure will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

The description herein makes reference to the accompanying drawingswherein like reference numerals refer to like parts throughout theseveral views.

FIG. 1 is a schematic diagram illustrating an example fuel cell systemin accordance with an embodiment of the present disclosure.

FIG. 2 is a schematic diagram illustrating an example cross section of afuel cell system in accordance with an embodiment of the presentdisclosure.

FIGS. 3A and 3B are plots illustrating XRD results for an examplenickelate cathode composition before and after aging, respectively.

FIGS. 4A and 4B are plots illustrating XRD results for an examplenickelate composite cathode composition before and after aging,respectively.

FIGS. 5A and 5B are plots illustrating XRD results for another examplenickelate composite cathode composition before and after aging,respectively.

FIGS. 6A and 6B are plots illustrating XRD results for another examplenickelate composite cathode composition before and after aging,respectively.

FIGS. 7A and 7B are plots illustrating XRD results for another examplenickelate composite cathode composition before and after aging,respectively.

FIGS. 8A and 8B are plots illustrating XRD results for another examplenickelate composite cathode composition before and after aging,respectively.

FIGS. 9A and 9B are transmission electron microscopy (TEM) images fortwo different example cathode compositions.

FIGS. 10A to 10C are additional transmission electron microscopy (TEM)images for example cathode compositions.

FIGS. 11A and 11B are additional transmission electron microscopy (TEM)images for two different example cathode compositions.

FIGS. 12A and 12B are scanning electron microscopy (SEM) images for twodifferent example cathode compositions.

FIG. 13 is a plot illustrating the results of a short-term durabilitytest of example cathode asymmetric button cells.

FIG. 14 is another plot illustrating the results of a short-termdurability test of example cathode asymmetric button cells.

FIGS. 15 and 16 are plots of results from long term durability testcarried out on example nickelate and/or nickelate composite cathodes.

FIG. 17 is bar chart showing cathode polarization for various examplenickelate composite cathodes.

FIG. 18 is bar chart showing cathode polarization for various additionalexample nickelate composite cathodes.

Referring to the drawings, some aspects of a non-limiting example of afuel cell system in accordance with an embodiment of the presentdisclosure is schematically depicted. In the drawing, various features,components and interrelationships therebetween of aspects of anembodiment of the present disclosure are depicted. However, the presentdisclosure is not limited to the particular embodiments presented andthe components, features and interrelationships therebetween as areillustrated in the drawings and described herein.

DETAILED DESCRIPTION

In accordance with some aspects of the disclosure, one or moretechniques may be used to manage phase composition of a nickelatecathode to maintain favorable phases during fuel cell operation, toachieve relatively fine microstructure for higher triple phase boundary(i.e., increased site reaction density) and stronger cathode-interlayer(which may also be referred to as a barrier) later bonding throughaddition of an ionic phase, a second oxide material, a second and thirdoxide material, or an ionic phase and a second or a second and a thirdoxide material into the nickelate cathode. For example, as will bedescribed below, the composition of the ionic phase, the second oxidematerial, the second and third oxide materials, or the ionic phase andthe second or the second and the third oxide material may be variedbased on nickelate cathode composition to manage element diffusion(e.g., minimize material diffusion) to maintain favorable phases duringfuel cell operation. Furthermore, the particular nickelate compositionsof the cathode disclosed herein may exhibit lower ASR and/or betterperformance compared to other nickelates, such as, e.g., cathodescomprising a lanthanide nickelate having the general formulaLn₂NiO_(4+δ). Additionally, examples of the disclosure may also includea cathode barrier layer separating the nickelate composite cathode froman electrolyte in the fuel cell. The cathode barrier layer may be formedof a co-doped ceria and may be configured to manage material diffusion(e.g., out of the cathode) and increase cathode phase stability.

Examples of the disclosure may provide for one or more advantages. Insome examples, the nickelate composite cathode materials describedherein may be used to improve fuel cell system performance and reducecost by enabling the fuel cell system to be operated at temperatureslower than the operating temperature(s) of fuel cells using differentcathode materials. For example, by using the nickelate composite cathodematerials disclosed herein, a fuel cell may be operated at a temperaturebetween about 700 to about 900 degrees Celsius to generate electricityfrom a fuel source, such as, e.g., hydrogen, natural gas, or syngasfuel, with high performance and long term durability and cost reduction.In some examples during the operation of a fuel cell, the phasecomposition of nickelate composite cathodes may be managed to maintainone or more favorable phases to improve long term durability of the fuelcell through the addition of an ionic phase, a second oxide material, asecond and third oxide material, or an ionic material and a second or asecond and third oxide material with different composition. In someexamples, the phase composition of nickelate composite cathodes duringfuel cell operation may be managed to maintain favorable phase toimprove long term durability through the use of a cathode barrier layerwith a composition selected based on the composition of the nickelatecathode. Examples of the disclosure may provide for improved nickelatecathode microstructure to increase triple phase boundary for both lowerASR and long term stability through the addition of an ionic phase, asecond oxide material, a second and third oxide material, or an ionicmaterial and a second or a second and third oxide material to thenickelate of the cathode. In some examples, the disclosed nickelatecomposite cathode may retain the low ASR of nickelate cathodes at alower operating temperature thereby improving system performance andlong term durability of the fuel cell, leading to cost reductions. Insome examples, the disclosure relates to approaches for improving thebond or attachment of a nickelate cathode with a cathode barrier layer,which may allow for reduced interface polarization, bond strength,improved long term cathode reliability, or any of the above. Multipleelectrochemical tests have been performed on example nickelate compositecathodes of this disclosure and the test demonstrated improved long termdurability and a reduction in degradation rate. Examples of thedisclosure may provide for other additional advantages, such as, e.g.,those apparent from the description herein.

FIG. 1 is a schematic diagram illustrating an example fuel cell system10 in accordance with an embodiment of the present disclosure. As shownin FIG. 1, fuel cell system 10 includes a plurality of electrochemicalcells 12 (or “individual fuel cells”) formed on substrate 14.Electrochemical cells 12 are coupled together in series by interconnect16. Fuel cell system 10 is a segmented-in-series arrangement depositedon a flat porous ceramic tube, although it will be understood that thepresent disclosure is equally applicable to segmented-in-seriesarrangements on other substrates, such on a circular porous ceramictube. In various embodiments, fuel cell system 10 may be an integratedplanar fuel cell system or a tubular fuel cell system.

Each electrochemical cell 12 includes an oxidant side 18 and a fuel side20. The oxidant is generally air, but could also be pure oxygen (O₂) orother oxidants, e.g., including dilute air for fuel cell systems havingair recycle loops, and is supplied to electrochemical cells 12 fromoxidant side 18. Substrate 14 may be specifically engineered, e.g., suchthat the porous ceramic material is stable at fuel cell operationconditions and chemically compatible with other fuel cell materials. Inother embodiments, substrate 14 may be a surface-modified material,e.g., a porous ceramic material having a coating or other surfacemodification, e.g., configured to prevent or reduce interaction betweenelectrochemical cell 12 layers and substrate 14. A fuel, such as areformed hydrocarbon fuel, e.g., synthesis gas, is supplied toelectrochemical cells 12 from fuel side 20 via channels (not shown) inporous substrate 14. Although air and synthesis gas reformed from ahydrocarbon fuel may be employed in some examples, it will be understoodthat electrochemical cells using other oxidants and fuels may beemployed without departing from the scope of the present disclosure,e.g., pure hydrogen and pure oxygen. In addition, although fuel issupplied to electrochemical cells 12 via substrate 14, it will beunderstood that in other embodiments, the oxidant may be supplied to theelectrochemical cells via a porous substrate.

FIG. 2 is a diagram illustrating an example configuration of anelectrochemical fuel cell 12 in accordance with an embodiment of thepresent disclosure. Electrochemical fuel cell 12 may be formed of aplurality of layers screen printed onto substrate 14 (or porous anodebarrier layer). This screen printing may include a process whereby awoven mesh has openings through which the fuel cell layers are depositedonto substrate 14. The openings of the screen determine the length andwidth of the printed layers. Screen mesh, wire diameter, ink solidsloading and ink rheology may determine the thickness of the printedlayers.

Electrochemical cell 12 includes cathode current collector 22, cathode24, cathode barrier layer 26, electrolyte 28, and anode 30. In one form,each of the respective components may be a single layer or may be formedof any number of sub-layers. It will be understood that FIG. 2 is notnecessarily to scale. For example, vertical dimensions are exaggeratedfor purposes of clarity of illustration. Additionally, one or more otherlayers (such as, e.g., a porous anode barrier, ceramic seal, andchemical barrier) may be present in other examples, such as, e.g., ananode current collector which may be disposed between the anode 30 andthe porous substrate 14 (not shown).

In electrochemical cell 12, anode 30 conducts free electrons to cathodecurrent collector 22 via interconnect 16 (shown in FIG. 1). Cathodecurrent collector 22 conducts the electrons to cathode 24. Interconnect16 (shown in FIG. 1) is electrically coupled to anode 30 and to cathodecurrent collector 22 of adjacent electrochemical cells.

Interconnects 16 (FIG. 1) for solid oxide fuel cells (SOFC) arepreferably electrically conductive in order to transport electrons fromone electrochemical cell to another; mechanically and chemically stableunder both oxidizing and reducing environments during fuel celloperation; and, nonporous in order to prevent diffusion of the fueland/or oxidant through the interconnect. If the interconnect is porous,fuel may diffuse to the oxidant side and burn, resulting in local hotspots that may result in a reduction of fuel cell life, e.g., due todegradation of materials and mechanical failure, as well as reducedefficiency of the fuel cell system. Similarly, the oxidant may diffuseto the fuel side, resulting in burning of the fuel. Severe interconnectleakage may significantly reduce the fuel utilization and performance ofthe fuel cell, or cause catastrophic failure of fuel cells or stacks.

Interconnect 16 may be formed of a precious metal including Ag, Pd, Auand/or Pt and/or alloys thereof, although other materials may beemployed without departing from the scope of the present disclosure. Forexample, in other embodiments, it is alternatively contemplated thatother materials may be employed, including precious metal alloys, suchas Ag—Pd, Ag—Au, Ag—Pt, Au—Pd, Au—Pt, Pt—Pd, Ag—Au—Pd, Ag—Au—Pt,Ag—Au—Pd—Pt and/or binary, ternary, quaternary alloys in the Pt—Pd—Au—Agfamily, inclusive of alloys having minor non-precious metal additions,cermets composed of a precious metal, precious metal alloy, and an inertceramic phase, such as alumina, or ceramic phase with minimum ionicconductivity which will not create significant parasitics, such as YSZ(yttria stabilized zirconia, also known as yttria doped zirconia, whereyttria doping is 3-8 mol %, preferably 3-5 mol %), ScSZ (scandiastabilized zirconia, where scandia doping is 4-10 mol %, preferably 4-6mol %), doped ceria, and/or conductive ceramics, such as conductiveperovskites with A or B-site substitutions or doping to achieve adequatephase stability and/or sufficient conductivity as an interconnect, e.g.,including at least one of doped strontium titanate (such asLa_(x)Sr_(1-x)TiO_(3-δ), x=0.1 to 0.3), LSCM(La_(1-x)Sr_(x)Cr_(1-y)Mn_(y)O₃, x=0.1 to 0.3 and y=0.25 to 0.75), dopedyttrium chromites (such as Y_(1-x)Ca_(x)CrO_(3-δ), x=0.1-0.3) and/orother doped lanthanum chromites (such as La_(1-x)Ca_(x)CrO_(3-δ), wherex=0.15-0.3), and conductive ceramics, such as doped strontium titanate,doped yttrium chromites, LSCM (La_(1-x)Sr_(x)Cr_(1-y)Mn_(y)O₃), andother doped lanthanum chromites. In one example, interconnect 16 may beformed of y(Pd_(x)Pt_(1-x))-(1-y)YSZ. Where x is from 0 to 1 in weightratio; preferably x is in the range of 0 to 0.5 for lower hydrogen flux.y is from 0.35 to 0.80 in volume ratio; preferably y is in the range of0.4 to 0.6.

Anode 30 may be an electrode conductive layer formed of a nickel cermet,such as Ni—YSZ (e.g., where yttria doping in zirconia is 3-8 mol %,),Ni—ScSZ (e.g., where scandia doping is 4-10 mol %, preferably includinga second doping for example 1 mol % ceria for phase stability for 10 mol% scandia-ZrO₂) and/or Ni-doped ceria (such as Gd or Sm doping), dopedlanthanum chromite (such as Ca doping on A site and Zn doping on Bsite), doped strontium titanate (such as La doping on A site and Mndoping on B site), La_(1-x)Sr_(x)Mn_(y)Cr_(1-y)O₃ and/or Mn-based R—Pphases of the general formula a (La_(1-x)Sr_(x))_(n+1)Mn_(n)O_(3n+1)Alternatively, it is considered that other materials for anode 30 may beemployed such as cermets based in part or whole on precious metal.Precious metals in the cermet may include, for example, Pt, Pd, Au, Ag,and/or alloys thereof. The ceramic phase may include, for example, aninactive non-electrically conductive phase, including, for example, YSZ,ScSZ and/or one or more other inactive phases, e.g., having desired CTEin order to control the CTE of the layer to match the CTE of thesubstrate and electrolyte. In some embodiments, the ceramic phase mayinclude Al₂O₃ and/or a spinel such as NiAl₂O₄, MgAl₂O₄, MgCr₂O₄, andNiCr₂O₄. In other embodiments, the ceramic phase may be electricallyconductive, e.g., doped lanthanum chromite, doped strontium titanateand/or one or more forms of LaSrMnCrO and/or R—P phases of the generalformula (La_(1-x)Sr_(x))_(n+1)Mn_(n)O_(3n+1).

Electrolyte 28 may be made from a ceramic material. In one form, aproton and/or oxygen ion conducting ceramic, may be employed. In oneform, electrolyte 28 is formed of YSZ, such as 3YSZ and/or 8YSZ. Inother embodiments, electrolyte layer 28 may be formed of ScSZ, such as4ScSZ, 6ScSz and/or 10Sc1CeSZ in addition to or in place of YSZ. Inother embodiments, other materials may be employed. For example, it isalternatively considered that electrolyte 28 may be made of doped ceriaand/or doped lanthanum gallate. In any event, electrolyte 28 issubstantially impervious to diffusion there through of the fluids usedby fuel cell 10, e.g., synthesis gas or pure hydrogen as fuel, as wellas, e.g., air or O₂ as an oxidant, but allows diffusion of oxygen ionsor protons.

Cathode current collector 22 may be an electrode conductive layer formedof a conductive ceramic, for example, at least one of LaNi_(x)Fe_(1-x)O₃(such as, e.g., LaNi_(0.6)Fe_(0.4)O₃), La_(1-x)Sr_(x)MnO₃ (such asLa_(0.75)Sr_(0.25)MnO₃), La_(1-x)Sr_(x)CoO₃ and/or Pr_(1-x)Sr_(x)CoO₃,such as Pr_(0.8)Sr_(0.2)CoO₃. In other embodiments, Cathode currentcollector 22 may be formed of other materials, e.g., a precious metalcermet, although other materials may be employed without departing fromthe scope of the present disclosure. The precious metals in the preciousmetal cermet may include, for example, Pt, Pd, Au, Ag and/or alloysthereof. The ceramic phase may include, for example, YSZ, ScSZ andAl₂O₃, or other non-conductive ceramic materials as desired to controlthermal expansion. As will be described below, in some examples, cathodecurrent collector 22 may be formed of a conductive ceramic which ischemically compatible with the nickelate composite of cathode 24, suchas, e.g., LNF. For example, when the conductive ceramic is chemicallycompatible with the nickelate composite, substantially no chemicalreaction occurs when the two materials contact each other and there isno third phase formation.

In some examples, anode 30 may have a thickness of approximately 5-20microns, although other values may be employed without departing fromthe scope of the present disclosure. For example, it is considered thatin other embodiments, the anode layer may have a thickness in the rangeof approximately 5-40 microns. In yet other embodiments, differentthicknesses may be used, e.g., depending upon the particular anodematerial and application.

Electrolyte 28 may have a thickness of approximately 5-15 microns withindividual sub-layer thicknesses of approximately 5 microns minimum,although other thickness values may be employed without departing fromthe scope of the present disclosure. For example, it is considered thatin other embodiments, the electrolyte layer may have a thickness in therange of approximately 5-200 microns. In yet other embodiments,different thicknesses may be used, e.g., depending upon the particularmaterials and application.

Cathode 24 may have a thickness of approximately 3-30 microns, such as,e.g., approximately 5-10 microns, although other values may be employedwithout departing from the scope of the present disclosure. For example,it is considered that in other embodiments, the cathode layer may have athickness in the range of approximately 10-50 microns. In yet otherembodiments, different thicknesses may be used, e.g., depending upon theparticular cathode material and application.

Cathode current collector 22 may have a thickness of approximately 5-100microns, although other values may be employed without departing fromthe scope of the present disclosure. For example, it is considered thatin other embodiments, cathode current collector 22 may have a thicknessless than or greater than the range of approximately 5-100 microns. Inyet other embodiments, different thicknesses may be used, e.g.,depending upon the particular material and application.

In some examples, cathodes may be electronic conductors only. To improvethe cathode performance, separate ionic phase may be added to helpoxygen ion transport to triple phase boundary away from the electrolyteinterface. Some cathodes, such as nickelates, may be mixedionic-electronic conductor. Theoretically second ionic phase may not benecessary for these cathode materials. However, there are benefits forthese materials, such as nickelate, to form a composite cathode withsecond ionic phase. The benefits may include, but are not limited to,microstructure control to increase triple phase boundary, improvement ofinterface adhesion to increase triple phase boundary and reduceinterface ohmic resistance, or management of materials diffusion, suchas for nickelate.

In accordance with examples of the disclosure, cathode 24 may be formedof a nickelate composite material including a nickelate compound and anionic conductive material. The nickelate compound may comprise at leastone of Pr₂NiO₄, Nd₂NiO₄, (Pr_(u)Nd_(v))₂NiO₄, (Pr_(u)Nd_(v))₃Ni₂O₇,(Pr_(u)Nd_(v))₄Ni₃O₁₀, or (Pr_(u)Nd_(v)M_(w))₂NiO₄, where M is analkaline earth metal doped on an A-site of Pr and Nd, 0.05≦w≦0.3,0.9≦u+v≦1.1, preferably 0.95≦u+v≦1.0, 0.9≦u+v+w≦1.1, preferably0.95≦u+v+w≦1.0. The composite cathode formed from such nickelatecompounds may exhibit relatively low ASR and relatively high performancein terms of ASR or internal resistance of a fuel cell, e.g., compared toother nickelate compounds, such as, e.g., lanthanide nickelate havingthe general formula Ln₂NiO_(4+δ). The lower ASR, the higher performance.Higher ASR will result in higher heat loss and less power output. Insome examples, the nickelate composite cathode 24 may have an ASR ofapproximately 0.02 ohm-cm² at 1 bara and about 790 degrees Celsius orless and lower degradation rate.

The ionic conductive material of cathode 24 may be provided to improveone more properties of cathode 24, such as, e.g., increased triple phaseboundary, improved adhesion with cathode barrier layer, desired phaseconstitution, and reduced degradation of cathode layer 24 in a hightemperature operating environment compared to that of a cathode withonly the nickelate composition. The ionic conductive material maycomprise a first co-doped ceria with a general formula of(A_(x)B_(y))Ce_(1-x-y)O₂, where A and B of the first co-doped ceria arerare earth metals. In some examples, the one of A and B of the firstco-doped ceria is Pr or Nd. In some examples, A is Pr and B is Nd.Theoretically any rare earth metal can be selected for A or B. However,since there is Pr or Nd on A site of nickelate, Pr, or Nd, or Pr and Ndmay be preferred for A, or B or A and B for less materialinter-diffusion between nickelate and doped ceria.

In some examples, examples of the disclosure may control and managephase constitution in a nickelate composite cathode to maintain desiredphases for lower cathode or fuel cell degradation rate throughminimizing material diffusion. For example, the presence and compositionof the ionic conductive material may manage material interdiffusion andmaintain desired phase constitution in nickelate composite cathode 24.Nickelate has a general formulation, A_(n+1)B_(n)O_(3n+1) (where n=1, 2,3, etc.), and different phase with composition change. When n=1, thenickelate phase mentioned herein may not be stable under fuel celloperating conditions, and a rare earth metal in A site, such as Pr,tends to exsolutes from nickelate structure to form oxide. PrO_(x)oxide, which has higher CTE (e.g., about 19 ppm/K from room temperatureto about 900 degrees Celsuis), compared to nickelate compounds (e.g.,about 14 ppm/K). Continued exsolution of rare earth metal from the Asite may result in Ni rich in B site, and then Ni may exsolute from Bsite to form NiO which is known inactive catalyst. Both rare earth metaloxide (such as PrO_(x)) and NiO may be referred to as the third phase inthe composite cathode. The formation of third phases in nickelatecomposite cathode may change cathode microstructure and thermalexpansion to cause degradation due to reduction of triple phase boundaryand local cathode detachment from electrolyte or cathode barrier layerdue to thermal stress.

Therefore, rare earth metal oxide, such as PrO_(x), and NiO may not bedesired phases. Doped ceria may be a stable phase. When formingcomposite cathode with a nickelate compound, it can adsorb rare earthmetal oxide, such as PrO_(x), exsoluted from the nickelate to form solidsolution to avoid undesired phase formation. Especially if doped ceriaalready contains Pr, or Nd, or both Pr and Nd in its startingcomposition, it can slow down or hinder rare earth metal exulution fromnickelate. In this manner, in some examples, the ionic phase can managephase constitution in nickelate composite cathode, which can be achievedthrough controlling the composition of doped ceria and the amount ofdoped ceria added to nickelate.

Cathode 24 may include any suitable concentrations of nickelate compoundand ionic conductive compound. In some examples, cathode 24 may includeapproximately 10 weight percent (wt %) to approximately 95 wt % of thenickelate compound, such as, e.g., approximately 50 wt % toapproximately 70 wt % of the nickelate compound. In some examples,cathode 24 may include approximately 5 wt % to approximately 90 wt % ofthe ionic conductive compound, such as, e.g., approximately 20 wt % toapproximately 50 wt % of the ionic conductive compound. In someexamples, the preferred ionic phase ratio to nickelate may be about 10wt % to about 50 wt % depending on chemical composition of both dopedceria and nickelate compound. If the ionic phase is too low, it cannotadsorb all the rare earth metal oxide exsoluted from nickelate, whichwill form undesired third phase. If the ionic phase is too high, theexsolution of rare earth metal from nickelate may continue since ionicphase can adsorb more rare earth metal till B site rich nickelatecompound is formed, which may result in NiO exsolution from B site ofnickelate to form undesired third phase NiO. In some examples, cathode24 may consist of, consist essentially of, or comprise the nickelatecompound and ionic conductive compound.

The composition of cathode 24 may change from the compositionas-fabricated following operation of fuel cell 12 at a high temperature(e.g., greater than, e.g., about 700 degrees Celsius. For example, rareearth metal exsolution from nickelate may change cathode microstructureto reduce triple phase boundary and increase cathode CTE to causecathode detachment from electrolyte or cathode barrier layer in localarea. It may further cause B site rich nickelate cathode formation whichmay result in NiO exsolution from B site of nickelate cathode. All thesemicrostructural and phase changes may increase cathode ASR. In someexamples, cathode 24 is substantially free of oxide formed exsolutedA-site element and/or B-site element from the nickelate compoundfollowing operation at a temperature of approximately 790 degreesCelsius or greater after approximately 100 hours with degradation rateof about 0.03 ohm-cm²/1000 hr using symmetric button cell. In someexamples, cathode 24 includes diffused exsolute from the nickelate in aphase of the ionic conductive material following operation at atemperature of approximately 790 degrees Celsuis or greater after about100 to about 2200 hours with degradation rate of about 0.002 to about0.013 ohm-cm²/1000 hr using segmented-in-series cell design. In someexamples, fuel cell with cathode 24 exhibits an area specific resistance(ASR) of approximately 0.22 ohm-cm² or less following operation at atemperature of approximately 860 degrees Celsuis after approximately6600 hours.

Additionally, as shown in FIG. 2, electrochemical fuel cell 12 mayinclude a cathode barrier layer 26 between electrolyte 28 and cathode24. Cathode barrier layer 26 may be formed of a second co-doped ceriawith a general formula (A_(x)B_(y))Ce_(1-x-y)O₂, where at least one of Aor B of the second co-doped ceria is Pr or Nd. In some examples, A is Prand B is Nd.

In some examples, the function of cathode barrier layer 26 may be atleast twofold. First, the barrier layer can prevent chemical interactionbetween electrolyte 28 (e.g., Y or Sc stabilized zirconia) andnickelate. Without cathode barrier layer 26, the rare earth metal, suchas Pr, in nickelate may interact with electrolyte 28 to form Pr₂Zr₂O₇undesired phase to increase cell ASR under some conditions during fuelcell operation. Second, cathode barrier layer 26 can help to controlrare earth metal exsolution from nickelate compound based onconcentration difference (material tends to migrate from higherconcentration to lower concentration) to manage phase constitution incathode 24 to keep desired phases for lower degradation rate. Forexample, (Pr_(x)Nd_(y))Ce_(1-x-y)O₂ cathode barrier may be selected ascathode barrier layer 26 for (Pr_(u)Nd_(v))₂NiO₄, (Pr_(u)Nd_(v))₃Ni₂O₇,and (Pr_(u)Nd_(v))₄Ni₃O₁₀ cathode and composite cathode composed of suchnickelate compounds.

In one example of fuel cell 12, cathode 24 is formed of a compositenickelate having the general formula Pr₂NiO₄,-(A_(x)B_(y))Ce_(1-x-y)O₂,where A is rare earth metal (such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy,Yb, Y, Ho, Er, and the like), 0<x<0.5 (preferably 0.05≦x≦0.3), B isdifferent rare earth metal element from A (such as, e.g., Gd, La, Pr,Nd, Sm, Tb, Dy, Yb, Y, Ho, Er, and the like), and 0≦y≦0.5 (preferably0.05≦y≦0.3). In some examples, preferably one of A and B is Pr. Cathodebarrier layer 26 is formed of a composition having the general formula(A_(x)B_(y))Ce_(1-x-y)O₂, where A is rare earth metal (such as, e.g.,La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Ho, Er, and the like), 0<x<0.5(preferably 0.05≦x≦0.3), B is different rare earth metal element from A(such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Ho, Er, and thelike), and 0≦y<0.5 (preferably 0.05≦y≦0.3). In some examples, preferablyA or B is Pr. Cathode current collector 22 may be formed of a conductiveceramic which is chemically compatible with the nickelate composite ofcathode 24, such as, e.g., LNF. For example, when the conductive ceramicis chemically compatible with the nickelate composite, substantially nochemical reaction occurs when the two materials contact each other andthere is no third phase formation.

In another example, cathode 24 is formed of a composite nickelate havingthe general formula Nd₂NiO₄,-A_(x)Ce_((1-x))O₂, where A is rare earthmetal (such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Ho, Er, and thelike), and 0<x<1 (preferably 0.1≦x≦0.4). Cathode barrier layer 26 isformed of a composition having the general formula A_(x)Ce_((1-x))O₂,where A is rare earth metal (such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy,Yb, Y, Ho, Er, and the like), and 0<x<1 (preferably 0.1≦x≦0.4). Cathodecurrent collector 22 may be formed of a conductive ceramic which ischemically compatible with the nickelate composite of cathode 24, suchas, e.g., LNF. For example, when the conductive ceramic is chemicallycompatible with the nickelate composite, substantially no chemicalreaction occurs when the two materials contact each other and there isno third phase formation.

In another example, cathode 24 is formed of a composite nickelate havingthe general formula Nd₂NiO₄,-(A_(x)B_(y))Ce_(1-x-y)O₂, where A is rareearth metal (such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Ho, Er,and the like), 0<x<0.5 (preferably 0.05≦x≦0.3), B is different rareearth metal element from A (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy,Yb, Y, Ho, Er, and the like), and 0≦y≦0.5 (preferably 0.05≦y≦0.3). Insome examples, preferably one of A and B is Nd. Cathode barrier layer 26is formed of a composition having the general formula(A_(x)B_(y))Ce_(1-x-y)O₂, where A is rare earth metal (such as, e.g.,La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Ho, Er, and the like), 0<x<0.5(preferably 0.05≦x≦0.3), B is different rare earth metal element from A(such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Ho, Er, and thelike), and 0≦y<0.5 (preferably 0.05≦y≦0.3). In some example, preferablyA or B is Nd. Cathode current collector 22 may be formed of a conductiveceramic which is chemically compatible with the nickelate composite ofcathode 24, such as, e.g., LNF. For example, when the conductive ceramicis chemically compatible with the nickelate composite, substantially nochemical reaction occurs when the two materials contact each other andthere is no third phase formation.

In another example, cathode 24 is formed of a composite nickelate havingthe general formula (Pr_(u)Nd_(v))₂NiO₄,-(A_(x)B_(y))Ce_(1-x-y)O₂, whereA is rare earth metal (such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y,Ho, Er, and the like), 0≦x<0.5 (preferably 0.05≦x≦0.3), B is differentrare earth metal element from A (such as, e.g., Gd, La, Pr, Nd, Sm, Tb,Dy, Yb, Y, Ho, Er, and the like), 0≦y<0.5 (preferably 0.05≦y≦0.3),0<u<1, and 0<v<1 (preferably 0.25<u<0.75 and 0.25<v<0.75). In someexamples, preferably one of A and B are Pr and Nd. This may includefollowing cases: 1) A or B is Pr; 2) A or B is Nd; and 3) A is Pr and Bis Nd. Cathode barrier layer 26 is formed of a composition having thegeneral formula (A_(x)B_(y))Ce_(1-x-y)O₂, where A is rare earth metal(such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Ho, Er, and thelike), 0<x<0.5 (preferably 0.05≦x≦0.3), B is different rare earth metalelement from A (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Ho,Er, and the like), and 0≦y<0.5 (preferably 0.05≦y≦0.3). In someexamples, preferably A and B are Pr and Nd. This may include followingcases: 1) A or B is Pr; 2) A or B is Nd; and 3) A is Pr and B is Nd.Cathode current collector 22 may be formed of a conductive ceramic whichis chemically compatible with the nickelate composite of cathode 24,such as, e.g., LNF. For example, when the conductive ceramic ischemically compatible with the nickelate composite, substantially nochemical reaction occurs when the two materials contact each other andthere is no third phase formation.

In another example, cathode 24 is formed of a composite nickelate havingthe general formula (Pr_(u)Nd_(v))₃Ni₂O₇,-(A_(x)B_(y))Ce_(1-x-y)O₂,where A is rare earth metal (such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy,Yb, Y, Ho, Er, and the like), 0<x<0.5 (preferably 0.05≦x≦0.3), B isdifferent rare earth metal element from A (such as, e.g., Gd, La, Pr,Nd, Sm, Tb, Dy, Yb, Y, Ho, Er, and the like), 0≦y<0.5 (preferably0.05≦y≦0.3), 0<u<1, and 0<v<1 (preferably 0.25<u<0.75 and 0.25<v<0.75).In some examples, preferably one of A and B are Pr and Nd. This mayinclude following cases: 1) A or B is Pr; 2) A or B is Nd; and 3) A isPr and B is Nd. Cathode barrier layer 26 is formed of a compositionhaving the general formula (A_(x)B_(y))Ce_(1-x-y)O₂, where A is rareearth metal (such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Ho, Er,and the like), 0<x<0.5 (preferably 0.05≦x≦0.3), B is different rareearth metal from A (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y,Ho, Er, and the like), and 0≦y<0.5 (preferably 0.05≦y≦0.3). In someexamples, preferably A and B are Pr and Nd. This may include followingcases: 1) A or B is Pr; 2) A or B is Nd; and 3) A is Pr and B is Nd.Cathode current collector 22 may be formed of a conductive ceramic whichis chemically compatible with the nickelate composite of cathode 24,such as, e.g., LNF. For example, when the conductive ceramic ischemically compatible with the nickelate composite, substantially nochemical reaction occurs when the two materials contact each other andthere is no third phase formation.

In another example, cathode 24 is formed of a composite nickelate havingthe general formula (Pr_(u)Nd_(v))₄Ni₃O₁₀,-(A_(x)B_(y))Ce_(1-x-y)O₂,where A is rare earth metal (such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy,Yb, Y, Ho, Er, and the like), 0<x<0.5 (preferably 0.05≦x≦0.3), B isdifferent rare earth metal element from A (such as, e.g., Gd, La, Pr,Nd, Sm, Tb, Dy, Yb, Y, Ho, Er, and the like), and 0≦y<0.5 (preferably0.05≦y≦0.3). In some examples, preferably one of A and B are Pr and Nd.This may include following cases: 1) A or B is Pr; 2) A or B is Nd; and3) A is Pr and B is Nd. Cathode barrier layer 26 is formed of acomposition having the general formula (A_(x)B_(y))Ce_(1-x-y)O₂, where Ais rare earth metal (such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y,Ho, Er, and the like), 0<x<0.5 (preferably 0.05≦x≦0.3), B is differentrare earth metal from A (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb,Y, Ho, Er, and the like), 0≦y<0.5 (preferably 0.05≦y≦0.3), 0<u<1, and0<v<1 (preferably 0.25<u<0.75 and 0.25<v<0.75. In some examples,preferably A and B are Pr and Nd. This may include following cases: 1) Aor B is Pr; 2) A or B is Nd; and 3) A is Pr and B is Nd. Cathode currentcollector 22 may be formed of a conductive ceramic which is chemicallycompatible with the nickelate composite of cathode 24, such as, e.g.,LNF. For example, when the conductive ceramic is chemically compatiblewith the nickelate composite, substantially no chemical reaction occurswhen the two materials contact each other and there is no third phaseformation.

In another example, cathode 24 is formed of a composite nickelate havingthe general formula (Pr_(u)Nd_(v)M_(w))₂NiO₄,-(A_(x)B_(y))Ce_(1-x-y)O₂,where M is alkaline earth metal (such as, e.g., Sr, Ca, Ba, and thelike), 0<u<1 and 0<v<1 (preferably 0.3<u<0.7 and 0.3<v<0.7 or0.25<u<0.75 and 0.25<v<0.75), 0<w<0.3 (preferably, 0.05<w<0.15), A israre earth metal (such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Ho,Er, and the like), 0<x<0.5 (preferably 0.05≦x≦0.3), B is different rareearth metal element from A (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy,Yb, Y, Ho, Er, and the like), and 0≦y<0.5 (preferably 0.05≦y≦0.3). Insome examples, preferably one of A and B are Pr and Nd. This may includefollowing cases: 1) A or B is Pr; 2) A or B is Nd; and 3) A is Pr and Bis Nd. Cathode barrier layer 26 is formed of a composition having thegeneral formula (A_(x)B_(y))Ce_(1-x-y)O₂, where A is rare earth metal(such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Ho, Er, and thelike), 0<x<0.5 (preferably 0.05≦x≦0.3), B is different rare earth metalfrom A (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Ho, Er, andthe like), and 0≦y<0.5 (preferably 0.05≦y≦0.3). In some examples,preferably A and B are Pr and Nd. This may include following cases: 1) Aor B is Pr; 2) A or B is Nd; and 3) A is Pr and B is Nd. Cathode currentcollector 22 may be formed of a conductive ceramic which is chemicallycompatible with the nickelate composite of cathode 24, such as, e.g.,LNF. For example, when the conductive ceramic is chemically compatiblewith the nickelate composite, substantially no chemical reaction occurswhen the two materials contact each other and there is no third phaseformation.

In another example, cathode 24 is formed of a composite nickelate havingthe general formula(Pr_(u)Nd_(v)M_(w))₂Ni_(1-z)N_(z)O₄-(A_(x)B_(y))Ce_(1-x-y)O₂, where M isalkaline earth metal (such as, e.g., Sr, Ca, Ba, and the like), 0<u<1,and 0<v<1 (preferably 0.25<u<0.75 and 0.25<v<0.75), 0<w<0.3 (preferably,0.05<w<0.15), N is transition metal (such as, e.g., Cu, Co, Mn, Fe, Cr,and the like), 0<z<0.5 (preferably 0.05<z<0.2), A is rare earth metal(such as La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Ho, Er, and the like),0<x<0.5 (preferably 0.05≦x≦0.3), B is different rare earth metal elementfrom A (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Ho, Er, andthe like), and 0≦y<0.5 (preferably 0.05≦y≦0.3). In some examples,preferably one of A and B are Pr and Nd. This may include followingcases: 1) A or B is Pr; 2) A or B is Nd; and 3) A is Pr and B is Nd.Cathode barrier layer 26 may be formed of a composition having thegeneral formula (A_(x)B_(y))Ce_(1-x-y)O₂, where A is rare earth metal(such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Ho, Er, and thelike), 0<x<0.5 (preferably 0.05≦x≦0.3), B is different rare earth metalfrom A (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Ho, Er, andthe like), and 0≦y<0.5 (preferably 0.05≦y≦0.3). In some examples,preferably A and B are Pr and Nd. This may include following cases: 1) Aor B is Pr; 2) A or B is Nd; and 3) A is Pr and B is Nd. Cathode currentcollector 22 may be formed of a conductive ceramic which is chemicallycompatible with the nickelate composite of cathode 24, such as, e.g.,LNF. For example, when the conductive ceramic is chemically compatiblewith the nickelate composite, substantially no chemical reaction occurswhen the two materials contact each other and there is no third phaseformation.

In another example, cathode 24 is formed of a composite nickelate havingthe general formula Ln₂NiO₄-A_(1-x)B_(x)O_(y), where Ln is a rare earthmetal except for La (such as, e.g., Pr, Nd, Sm, etc. and preferably Pr),A is a rare earth metal except for Ce (such as, e.g., La, Pr, Nd, Gd,Sm, Tb, Dy, Yb, Y, Ho, Er, etc.), B is a rare earth metal which is bothdifferent from A and exclusive of Ce (such as, e.g., Gd, La, Pr, Nd, Sm,Tb, Dy, Yb, Y, Ho, Er, etc.), 0≦x<1 (preferably 0.25≦x≦0.75), 1.5≦y≦2.0(preferably 1.8≦y≦2.0) and preferably one of A and B is Pr. Cathodebarrier layer 26 is formed of a composition having the general formulaC_(w)D_(z)Ce_((1-w-z))O_(2-δ), where C is rare earth metal (such as,e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.), 0<w<0.75(preferably 0.05≦w≦0.5), D is a rare earth metal which is different fromC (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.),0≦z<0.75 (preferably 0.05≦z≦0.5), and δ is an oxygent stoichiometrydependent on the amount of w+z, 0≦δ<0.5 (preferably 0≦δ≦0.3). Thecathode current collector 22 may be formed of a conductive oxidematerial which is chemically compatible with a nickelate compositecathode, such as, e.g., LNF and LSM, etc., such that no chemicalreaction occurs when the two materials contact each other and there isno third phase formation.

In another example, cathode 24 is formed of a composite nickelate havingthe general formula (Ln_(u)M1_(v))₂NiO₄-A_(1-x)B_(x)O_(y), where Ln is arare earth metal except for La (such as, e.g., Pr, Nd, Sm, etc.), 0<u≦1(preferably 0.25≦u≦0.75), M1 is a rare earth metal which is bothdifferent from Ln and exclusive of La (such as, e.g., Pr, Nd, Sm, Gd,Tb, Dy, Yb, Y, Ho, Er, etc.), 0<v<1 (preferably 0.25≦v≦0.75),0.9≦u+v<1.1 (preferably 0.95≦u+v≦1.05), preferably one of Ln and M1 isPr, A is a rare earth metal except for Ce (such as, e.g., La, Pr, Nd,Gd, Sm, Tb, Dy, Yb, Y, Ho, Er, etc.), B is a rare earth metal which isboth different from A and exclusive of Ce (such as, e.g., Gd, La, Pr,Nd, Sm, Tb, Dy, Yb, Y, Ho, Er, etc.), 0≦x<1 (preferably 0.25≦x≦0.75),1.5≦y≦2.0 (preferably 1.8≦y≦2.0) and preferably one of A and B is Pr.Cathode barrier layer 26 is formed of a composition having the generalformula C_(w)D_(z)Ce_((1-w-z))O_(2-δ), where C is rare earth metal (suchas, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.), 0<w<0.75(preferably 0.05≦w≦0.5), D is a rare earth metal which is different fromC (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.),0≦z<0.75 (preferably 0.05≦z≦0.5), and δ is an oxygent stoichiometrydependent on the amount of w+z, 0≦δ<0.5 (preferably 0≦δ≦0.3). Thecathode current collector 22 may be formed of a conductive oxidematerial which is chemically compatible with a nickelate compositecathode, such as, e.g., LNF and LSM, etc., such that no chemicalreaction occurs when the two materials contact each other and there isno third phase formation.

In another example, cathode 24 is formed of a composite nickelate havingthe general formula(Ln_(u)M1_(v))_(n+1)Ni_(n)O_(3n+1)-A_(1-x)B_(x)O_(y), where Ln is a rareearth metal except for La (such as, e.g., Pr, Nd, Sm, etc.), 0<u≦1(preferably 0.25≦u≦0.75), M1 is a rare earth metal which is bothdifferent from Ln and exclusive of La (such as, e.g., Pr, Nd, Sm, Gd,Tb, Dy, Yb, Y, Ho, Er, etc.), 0<v<1 (preferably 0.25≦v≦0.75),0.9≦u+v<1.1 (preferably 0.95≦u+v≦1.05), 1<n (preferably 1<n≦3),preferably one of Ln and M1 is Pr, A is a rare earth metal except for Ce(such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Ho, Er, etc.), B is arare earth metal which is both different from A and exclusive of Ce(such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Ho, Er, etc.), 0≦x<1(preferably 0.25≦x≦0.75), 1.5≦y≦2.0 (preferably 1.8≦y≦2.0) andpreferably one of A and B is Pr. Cathode barrier layer 26 is formed of acomposition having the general formula C_(w)D_(z)Ce_((1-w-z))O_(2-δ),where C is rare earth metal (such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy,Yb, Y, Sc, Ho, Er, etc.), 0<w<0.75 (preferably 0.05≦w≦0.5), D is a rareearth metal which is different from C (such as, e.g., Gd, La, Pr, Nd,Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.), 0≦z<0.75 (preferably 0.05≦z≦0.5),and δ is an oxygent stoichiometry value dependent on the amount of w+z,0≦δ<0.5 (preferably 0≦δ≦0.3). The cathode current collector 22 may beformed of a conductive oxide material which is chemically compatiblewith a nickelate composite cathode, such as, e.g., LNF and LSM, etc.,such that no chemical reaction occurs when the two materials contacteach other and there is no third phase formation.

In another example, cathode 24 is formed of a composite nickelate havingthe general formula(Ln_(u)M1_(v))_(n+1)(Ni_(1-t)N_(t))_(n)O_(3n+1)-A_(1-x)B_(x)O_(y), whereLn is a rare earth metal except for La (such as, e.g., Pr, Nd, Sm,etc.), 0<u≦1 (preferably 0.25≦u≦0.75), M1 is a rare earth metal which isboth different from Ln and exclusive of La (such as, e.g., Pr, Nd, Sm,Gd, Tb, Dy, Yb, Y, Ho, Er, etc.), 0≦v<1 (preferably 0.25≦v≦0.75),0.9≦u+v<1.1 (preferably 0.95≦u+v≦1.05), preferably one of Ln and M1 isPr, N is one or more transition metals (such, e.g., Mn, Fe, Cu, Ti, Cr,Co, V, Zn, etc.), 0<t≦0.5 (preferably 0.02≦t≦0.3), 1≦n (preferably1≦n≦3), preferably N is Cu, A is a rare earth metal except for Ce (suchas, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Ho, Er, etc.), B is a rareearth metal which is both different from A and exclusive of Ce (such as,e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Ho, Er, etc.), 0≦x<1(preferably 0.25≦x≦0.75), 1.5≦y≦2.0 (preferably 1.8≦y≦2.0) andpreferably one of A and B is Pr. Cathode barrier layer 26 is formed of acomposition having the general formula C_(w)D_(z)Ce_((1-w-z))O_(2-δ),where C is rare earth metal (such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy,Yb, Y, Sc, Ho, Er, etc.), 0<w<0.75 (preferably 0.05≦w≦0.5), D is a rareearth metal which is different from C (such as, e.g., Gd, La, Pr, Nd,Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.), 0≦z<0.75 (preferably 0.05≦z≦0.5),and δ is an oxygent stoichiometry value dependent on the amount of w+z,0≦δ<0.5 (preferably 0≦δ≦0.3). The cathode current collector 22 may beformed of a conductive oxide material which is chemically compatiblewith a nickelate composite cathode, such as, e.g., LNF and LSM, etc.,such that no chemical reaction occurs when the two materials contacteach other and there is no third phase formation.

In another example, cathode 24 is formed of a composite nickelate havingthe general formulaLn₂NiO₄-A_(1-x)B_(x)O_(y)—C_(w)D_(z)Ce_((1-w-z))O_(2-δ), where Ln is arare earth metal (such as, e.g., La, Pr, Nd, Sm, etc. and preferablyPr), A is a rare earth metal except for Ce (such as, e.g., La, Pr, Nd,Gd, Sm, Tb, Dy, Yb, Y, Ho, Er, etc.), B is a rare earth metal which isboth different from A and exclusive of Ce (such as, e.g., Gd, La, Pr,Nd, Sm, Tb, Dy, Yb, Y, Ho, Er, etc.), 0≦x<1 (preferably 0.25≦x≦0.75),1.5≦y≦2.0 (preferably 1.8≦y≦2.0) and preferably one of A and B is Pr, Cis rare earth metal (such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y,Sc, Ho, Er, etc.), 0<w<0.75 (preferably 0.05≦w≦0.5), D is a rare earthmetal which is different from C (such as, e.g., Gd, La, Pr, Nd, Sm, Tb,Dy, Yb, Y, Sc, Ho, Er, etc.), 0≦z<0.75 (preferably 0.05≦z≦0.5), δ is anoxygent stoichiometry dependent on the amount of w+z, 0≦δ<0.5(preferably 0≦δ≦0.3), and preferably one of C and D is Pr. Cathodebarrier layer 26 is formed of a composition having the general formulaC_(w)D_(z)Ce_((1-w-z))O_(2-δ), where C is rare earth metal (such as,e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.), 0<w<0.75(preferably 0.05≦w≦0.5), D is a rare earth metal which is different fromC (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.),0≦z<0.75 (preferably 0.05≦z≦0.5), and δ is an oxygent stoichiometrydependent on the amount of w+z, 0≦δ<0.5 (preferably 0≦δ≦0.3). Thecathode current collector 22 may be formed of a conductive oxidematerial which is chemically compatible with a nickelate compositecathode, such as, e.g., LNF and LSM, etc., such that no chemicalreaction occurs when the two materials contact each other and there isno third phase formation.

In another example, cathode 24 is formed of a composite nickelate havingthe general formula(Ln_(u)M1_(v))₂NiO₄-A_(1-x)B_(x)O_(y)—C_(w)D_(z)Ce_((1-w-z))O_(2-δ),where Ln is a rare earth metal (such as, e.g., La, Pr, Nd, Sm, etc.),0<u≦1 (preferably 0.25≦u≦0.75), M1 is a rare earth metal different fromLn (such as, e.g., La, Pr, Nd, Sm, Gd, Tb, Dy, Yb, Y, Ho, Er, etc.),0<v<1 (preferably 0.25≦v≦0.75), 0.9≦u+v<1.1 (preferably 0.95≦u+v≦1.05),preferably one of Ln and M1 is Pr, A is a rare earth metal except for Ce(such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Ho, Er, etc.), B is arare earth metal which is both different from A and exclusive of Ce(such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Ho, Er, etc.), 0≦x<1(preferably 0.25≦x≦0.75), 1.5≦y≦2.0 (preferably 1.8≦y≦2.0) andpreferably one of A and B is Pr, C is rare earth metal (such as, e.g.,La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.), 0<w<0.75(preferably 0.05≦w≦0.5), D is a rare earth metal which is different fromC (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.),0≦z<0.75 (preferably 0.05≦z≦0.5), and δ is an oxygent stoichiometrydependent on the amount of w+z, 0≦δ<0.5 (preferably 0≦δ≦0.3), andpreferably one of C and D is Pr. Cathode barrier layer 26 is formed of acomposition having the general formula C_(w)D_(z)Ce_((1-w-z))O_(2-δ),where C is rare earth metal (such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy,Yb, Y, Sc, Ho, Er, etc.), 0<w<0.75 (preferably 0.05≦w≦0.5), D is a rareearth metal which is different from C (such as, e.g., Gd, La, Pr, Nd,Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.), 0≦z<0.75 (preferably 0.05≦z≦0.5),and δ is an oxygent stoichiometry dependent on the amount of w+z,0≦δ<0.5 (preferably 0≦δ≦0.3). The cathode current collector 22 may beformed of a conductive oxide material which is chemically compatiblewith a nickelate composite cathode, such as, e.g., LNF and LSM, etc.,such that no chemical reaction occurs when the two materials contacteach other and there is no third phase formation.

In another example, cathode 24 is formed of a composite nickelate havingthe general formula(Ln_(u)M1_(v))_(n+1)Ni_(n)O_(3n+1)-A_(1-x)B_(x)O_(y)-13C_(w)D_(z)Ce_((1-w-z))O_(2-δ), where Ln is a rare earth metal (such as,e.g., La, Pr, Nd, Sm, etc.), 0<u≦1 (preferably 0.25≦u≦0.75), M1 is arare earth metal different from Ln (such as, e.g., La, Pr, Nd, Sm, Gd,Tb, Dy, Yb, Y, Ho, Er, etc.), 0<v<1 (preferably 0.25≦v≦0.75),0.9≦u+v<1.1 (preferably 0.95≦u+v≦1.05), 1<n (preferably 1<n≦3),preferably one of Ln and M1 is Pr, A is a rare earth metal except for Ce(such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Ho, Er, etc.), B is arare earth metal which is both different from A and exclusive of Ce(such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Ho, Er, etc.), 0≦x<1(preferably 0.25≦x≦0.75), 1.5≦y≦2.0 (preferably 1.8≦y≦2.0) andpreferably one of A and B is Pr, C is rare earth metal (such as, e.g.,La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.), 0<w<0.75(preferably 0.05≦w≦0.5), D is a rare earth metal which is different fromC (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.),0≦z<0.75 (preferably 0.05≦z≦0.5), and δ is an oxygent stoichiometrydependent on the amount of w+z, 0≦δ<0.5 (preferably 0≦δ≦0.3), andpreferably one of C and D is Pr.

Cathode barrier layer 26 is formed of a composition having the generalformula C_(w)D_(z)Ce_((1-w-z))O_(2-δ), where C is rare earth metal (suchas, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.), 0<w<0.75(preferably 0.05≦w≦0.5), D is a rare earth metal which is different fromC (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.),0≦z<0.75 (preferably 0.05≦z≦0.5), and δ is an oxygent stoichiometryvalue dependent on the amount of w+z, 0≦δ<0.5 (preferably 0≦δ≦0.3). Thecathode current collector 22 may be formed of a conductive oxidematerial which is chemically compatible with a nickelate compositecathode, such as, e.g., LNF and LSM, etc., such that no chemicalreaction occurs when the two materials contact each other and there isno third phase formation.

In another example, cathode 24 is formed of a composite nickelate havingthe general formula(Ln_(u)M1_(v))_(n+1)(Ni_(1-t)N_(t))_(n)O_(3n+1)-A_(1-x)B_(x)O_(y)—C_(w)D_(z)Ce_((1-w-z))O_(2-δ),where Ln is a rare earth metal (such as, e.g., La, Pr, Nd, Sm, etc.),0<u≦1 (preferably 0.25≦u≦0.75), M1 is a rare earth metal different fromLn (such as, e.g., La, Pr, Nd, Sm, Gd, Tb, Dy, Yb, Y, Ho, Er, etc.),0≦v<1 (preferably 0.25≦v≦0.75), 0.9≦u+v<1.1 (preferably 0.95≦u+v≦1.05),preferably one of Ln and M1 is Pr, N is one or more transitions metals(such as, e.g., Mn, Fe, Cu, Ti, Cr, Co, V, Zn, etc.), 0≦t≦0.5(preferably 0.02≦t≦0.3), 1≦n (preferably 1≦n≦3), preferably N is Cu, Ais a rare earth metal except for Ce (such as, e.g., La, Pr, Nd, Gd, Sm,Tb, Dy, Yb, Y, Ho, Er, etc.), B is a rare earth metal which is bothdifferent from A and exclusive of Ce (such as, e.g., Gd, La, Pr, Nd, Sm,Tb, Dy, Yb, Y, Ho, Er, etc.), 0≦x<1 (preferably 0.25≦x≦0.75), 1.5≦y≦2.0(preferably 1.8≦y≦2.0) and preferably one of A and B is Pr, C is rareearth metal (such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Sc, Ho,Er, etc.), 0<w<0.75 (preferably 0.05≦w≦0.5), D is a rare earth metalwhich is different from C (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy,Yb, Y, Sc, Ho, Er, etc.), 0≦z<0.75 (preferably 0.05≦z≦0.5), and δ is anoxygent stoichiometry dependent on the amount of w+z, 0≦δ<0.5(preferably 0≦δ≦0.3), and preferably one of C and D is Pr. Cathodebarrier layer 26 is formed of a composition having the general formulaC_(w)D_(z)Ce_((1-w-z))O_(2-δ), where C is rare earth metal (such as,e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.), 0<w<0.75(preferably 0.05≦w≦0.5), D is a rare earth metal which is different fromC (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.),0≦z<0.75 (preferably 0.05≦z≦0.5), and δ is an oxygent stoichiometryvalue dependent on the amount of w+z, 0≦δ<0.5 (preferably 0≦δ≦0.3). Thecathode current collector 22 may be formed of a conductive oxidematerial which is chemically compatible with a nickelate compositecathode, such as, e.g., LNF and LSM, etc., such that no chemicalreaction occurs when the two materials contact each other and there isno third phase formation.

In another example, cathode 24 is formed of a composite nickelate havingthe general formula(Ln_(u)M1_(v)M2_(s))_(n+1)(Ni_(1-t)N_(t))_(n)O_(3n+1)-A_(1-x)B_(x)O_(y),where Ln is a rare earth metal except for La (such as, e.g., Pr, Nd, Sm,etc.), 0<u≦1 (preferably 0.25≦u≦0.75), M1 is a rare earth metal which isboth different from Ln and exclusive of La (such as, e.g., Pr, Nd, Sm,Gd, Tb, Dy, Yb, Y, Ho, Er, etc.), 0≦v<1 (preferably 0.25≦v≦0.75), M2 isan alkaline earth metal (such as, e.g., Sr, Ca, Ba, etc.), 0<s<0.3(preferably 0.02≦s≦0.15), 0.9≦u+v+s<1.1 (preferably 0.95≦u+v+s≦1.05),preferably one of Ln and M1 is Pr and M2 is Sr, N is one or moretransitions metals (such as, e.g., Mn, Fe, Cu, Ti, Cr, Co, V, Zn, etc.),0≦t≦0.5 (preferably 0.02≦t≦0.3), 1 <n (preferably 1≦n≦3), preferably Nis Cu, A is a rare earth metal except for Ce (such as, e.g., La, Pr, Nd,Gd, Sm, Tb, Dy, Yb, Y, Ho, Er, etc.), B is a rare earth metal which isboth different from A and exclusive of Ce (such as, e.g., Gd, La, Pr,Nd, Sm, Tb, Dy, Yb, Y, Ho, Er, etc.), 0≦x<1 (preferably 0.25≦x≦0.75),1.5≦y≦2.0 (preferably 1.8≦y≦2.0) and preferably one of A and B is Pr.Cathode barrier layer 26 is formed of a composition having the generalformula C_(w)D_(z)Ce_((1-w-z))O_(2-δ), where C is rare earth metal (suchas, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.), 0<w<0.75(preferably 0.05≦w≦0.5), D is a rare earth metal which is different fromC (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.),0≦z<0.75 (preferably 0.05≦z≦0.5), and δ is an oxygent stoichiometryvalue dependent on the amount of w+z, 0≦δ<0.5 (preferably 0≦δ≦0.3). Thecathode current collector 22 may be formed of a conductive oxidematerial which is chemically compatible with a nickelate compositecathode, such as, e.g., LNF and LSM, etc., such that no chemicalreaction occurs when the two materials contact each other and there isno third phase formation.

In another example, cathode 24 is formed of a composite nickelate havingthe general formula(Ln_(u)M1_(v)M2_(s))_(n+1)(Ni_(1-t)N_(t))_(n)O_(3n+1)-A_(1-x)B_(x)O_(y)—C_(w)D_(z)Ce_((1-w-z))O_(2-δ),where Ln is a rare earth metal (such as, e.g., La, Pr, Nd, Sm, etc.),0<u≦1 (preferably 0.25≦u≦0.75), M1 is a rare earth metal different fromLn (such as, e.g., La, Pr, Nd, Sm, Gd, Tb, Dy, Yb, Y, Ho, Er, etc.),0≦v<1 (preferably 0.25≦v≦0.75), M2 is an alkaline earth metal (such as,e.g., Sr, Ca, Ba, etc.), 0<s<0.3 (preferably 0.02≦s≦0.15), 0.9≦u+v+s<1.1(preferably 0.95≦u+v+s≦1.05), preferably one of Ln and M1 is Pr and M2is Sr, N is one or more transitions metals (such as, e.g., Mn, Fe, Cu,Ti, Cr, Co, V, Zn, etc.), 0≦t≦0.5 (preferably 0.02≦t≦0.3), 1≦n(preferably 1≦n≦3), preferably N is Cu, A is a rare earth metal exceptfor Ce (such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Ho, Er, etc.),B is a rare earth metal which is both different from A and exclusive ofCe (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Ho, Er, etc.),0≦x<1 (preferably 0.25≦x≦0.75), 1.5≦y≦2.0 (preferably 1.8≦y≦2.0) andpreferably one of A and B is Pr, C is rare earth metal (such as, e.g.,La, Pr, Nd, Gd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.), 0<w<0.75(preferably 0.05≦w≦0.5), D is a rare earth metal which is different fromC (such as, e.g., Gd, La, Pr, Nd, Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.),0≦z<0.75 (preferably 0.05≦z≦0.5), and δ is an oxygent stoichiometrydependent on the amount of w+z, 0≦δ<0.5 (preferably 0≦δ≦0.3), andpreferably one of C and D is Pr. Cathode barrier layer 26 is formed of acomposition having the general formula C_(w)D_(z)Ce_((1-w-z))O_(2-δ),where C is rare earth metal (such as, e.g., La, Pr, Nd, Gd, Sm, Tb, Dy,Yb, Y, Sc, Ho, Er, etc.), 0<w<0.75 (preferably 0.05≦w≦0.5), D is a rareearth metal which is different from C (such as, e.g., Gd, La, Pr, Nd,Sm, Tb, Dy, Yb, Y, Sc, Ho, Er, etc.), 0≦z<0.75 (preferably 0.05≦z≦0.5),and δ is an oxygent stoichiometry value dependent on the amount of w+z,0≦δ<0.5 (preferably 0≦δ≦0.3). The cathode current collector 22 may beformed of a conductive oxide material which is chemically compatiblewith a nickelate composite cathode, such as, e.g., LNF and LSM, etc.,such that no chemical reaction occurs when the two materials contacteach other and there is no third phase formation.

EXAMPLES

Various experiments were carried out to evaluate one or more aspects ofexample cathode compositions and fuel cells employing such cathodecompositions in accordance with the disclosure. However, examples of thedisclosure are not limited to the experimental anode compositions.

In one instance, nickelate pellets formed of a material having theformula (Pr_(0.5)Nd_(0.5))₂NiO₄ were fabricated and then aged for about500 hours at approximately 870 degrees C. FIG. 3A illustrates XRDpatterns of the nickelate pellets as-fabricated. FIG. 3B illustrates XRDpatterns of the nickelate pellets after aging. As shown, PrO_(x) oxidethat had exsoluted from the nickelate was identified after aging.PrO_(x) is an undesired third phase since it may change cathodemicrostructure to reduce triple phase boundary and cause cathodedetachment in local area resulted from higher thermal stress due to CTEmismatch. Additionally, a small NiO peak was identified.

In another instance, nickelate composite pellets formed of a materialhaving the formula (Pr_(0.5)Nd_(0.5))₂NiO₄-15% GDC10 were fabricated andthen aged for about 500 hours at approximately 870 degrees C. FIG. 4Aillustrates XRD patterns of the nickelate composite pelletsas-fabricated. FIG. 4B illustrates XRD patterns of the nickelatecomposite pellets after aging. As shown, PrO_(x) oxide was not formed inthe aged nickelate composite since exsoluted Pr was diffused in GDCphase. However, a small NiO peak was identified after aging. Even thoughboth PrO_(x) and NiO may be undesired phases,(Pr_(0.5)Nd_(0.5))₂NiO₄-15% GDC10 composite cathode showed much improvedphase constitution after 500 hours of aging compared to(Pr_(0.5)Nd_(0.5))₂NiO₄ cathode due to significant reduction ofundesired phase.

In another instance, nickelate composite pellets formed of a materialhaving the formula (Pr_(0.5)Nd_(0.5))₂NiO₄-15%(Pr_(0.24)Nd_(0.21))Ce_(0.55)O₂ were fabricated and then aged for about500 hours at approximately 870 degrees C. FIG. 5A illustrates XRDpatterns of the nickelate composite pellets as-fabricated. FIG. 5Billustrates XRD patterns of the nickelate composite pellets after aging.As shown, third phase for both PrO_(x) and NiO was not identified in thematerial. Additionally, for the example composite cathode material, lessmaterial migration occurred during firing and aging. The identifiedphases were higher order nickelate (e.g., n=3) and doped ceria,(Pr,Nd,Ce)O_(x). In this example, (Pr_(0.5)Nd_(0.5))₂NiO₄-15%(Pr_(0.24)Nd_(0.21))Ce_(0.55)O₂, only higher order nickelate and dopedceria phases existed after 500 hours of aging at 870° C. showing furtherimproved phase constitution compared to (Pr_(0.5)Nd_(0.5))₂NiO₄-15%GDC10 composite cathode since both phases are desired phase. Asdescribed before, nickelates have different phases, n=1 (lower orderphase), 2, and 3 (higher order phase). All are desired phases.

In another instance, nickelate composite pellets having the formulaPNN5050-30 wt % (PrNd)O_(y) were fabricated and then aged for about 500hours at approximately 870 degrees C. FIG. 6A illustrates the XRDpatterns of the nickelate composite pellets as-fabricated. As shown,PrO_(x) oxide appeared in FIG. 6A due to the addition of the PrO_(x)phase. FIG. 6B illustrates the XRD patterns of the nickelate compositepellets after aging. As shown, the PrOx peak increased slightly and noNiO was identified in the material.

In another instance, nickelate composite pellets having the formulaPNN5050-30 wt % (PrNd)O_(y) were fabricated and then aged for about 500hours at approximately 870 degrees C. FIG. 6A illustrates the XRDpatterns of the nickelate composite pellets as-fabricated. As shown,PrO_(x) oxide appeared in FIG. 6A due to the addition of the PrO_(x)phase. FIG. 6B illustrates the XRD patterns of the nickelate compositepellets after aging. As shown, the PrO_(x) peak increased slightly andno NiO was identified in the material.

In another instance, nickelate composite pellets having the formulaPNN5050-30 wt % (PrNd)O_(y) were fabricated and then aged for about 500hours at approximately 870 degrees C. FIG. 6A illustrates the XRDpatterns of the nickelate composite pellets as-fabricated. As shown,PrO_(x) oxide appeared in FIG. 6A due to the addition of the PrO_(x)phase. FIG. 6B illustrates the XRD patterns of the nickelate compositepellets after aging. As shown, the PrO_(x) peak increased slightly andno NiO was identified in the material.

In another instance, nickelate composite pellets having the formula(Pr_(0.48),Nd_(0.52))₄Ni₃O₁₀(“PN4N3”)-30 wt % (PrNd)O_(y) werefabricated and then aged for about 500 hours at approximately 870degrees C. FIG. 7A illustrates the XRD patterns of the nickelatecomposite pellets as-fabricated. FIG. 7B illustrates the XRD patterns ofthe nickelate composite pellets after aging. As shown, no NiO peak orundesired phase was identified after aging Additionally, for thiscathode composition less material migration occurred during firing andaging of the nickelate composite pellets.

In another instance, nickelate composite pellets having the formulaPNN5050-15 wt % Pr₆O₁₁-15 wt % (Pr_(0.25)Nd_(0.21))Ce_(0.55)O_(x) werefabricated and then aged for about 500 hours at approximately 870degrees C. FIG. 8A illustrates the XRD patterns of the nickelatecomposite pellets as-fabricated. FIG. 8B illustrates the XRD patterns ofthe nickelate composite pellets after aging. As shown, neither impurityphase, PrO_(x) or NiO, was identified after aging. After aging, higherorder nickelate (n=3), doped ceria, and (Pr,Nd)O_(y) phases whereidentified.

FIG. 9A is a transmission electron microscopy (TEM) image of the example(Pr_(0.5)Nd_(0.5))₂NiO₄ material described above after being aged forabout 500 hours at approximately 870 degrees C. As shown in the image,(Pr,Nd)O_(x) oxide was identified, e.g., at locations 5, 6, 7, and 8 inFIG. 6A.

FIG. 9B is a TEM image of the example (Pr_(0.5)Nd_(0.5))₂NiO₄-15% GDC10composite material described above after being aged for about 500 hoursat approximately 870 degrees C. As shown in the image, Pr and Nddiffused into GDC and formed (Pr,Nd,Gd)CeO₂ solid solution, e.g., atlocations 5, 6, and 7 in FIG. 6B. The Pr and Nd in the solid solutionwas determined to be as high as about 29% and about 20%, respectively.Further, NiO was identified, e.g., as location 1 in FIG. 6B.

The TEM analysis of nickelate or nickelate composite cathode confirmedour hypothesis about rare earth metal exsolution from nickelate and itsdissolution into second ionic phase, doped ceria. The phase constitutionin cathode was able to be managed through selection of rare earth metalas dopant for ceria and doping level, as well as the addition level ofdoped ceria in composite cathode.

In another example, FIG. 10A is a TEM image of a PNN5050 cathodematerial as fabricated. FIG. 10B is a TEM image of a PNN5050 cathodematerial after cell operation for about 4000 hours and around 790degrees C. FIG. 10C is a TEM image of PNN5050 cathode material aftercell operation for about 2400 hours and about 870 degrees C. A NiO phasecan been seen at locations 1 to 7 and a PrOx phase at locations 8 to 14,representing a phase separation or decomposition. Additionally, thePNN5050 cathode material has a coarser microstructure after operationthan the microstructure of the as-fabricated PNN5050 cathode.

FIG. 11A is a TEM image of the example PNN5050-30 wt % (PrNd)O_(y)composite material described above after being aged for about 500 hoursat approximately 870 degrees C. As shown in the image, a NiO phase wasnot identified, a result which agrees with the XRD results.Additionally, Pr and Nd have exsoluted from the nickelate leading to theformation of (Pr,Nd)O_(y) seen at locations 1 to 5. This indicates thatthe PNN5050 phase may be changed into a higher order nickelatecomposition.

FIG. 11B is a TEM image of the example PN4N3-30 wt % (PrNd)O_(y)composite material described above after being aged for about 500 hoursat approximately 870 degrees C. As shown in the image, a NiO phase wasnot identified, a result which agrees with the XRD results.Additionally, the post-again TEM image indicates that the phases of theexample are the same as those originally mixed.

FIGS. 12A and 12B are a scanning electron microscopy (SEM) images of twoexample electrochemical cell structures (cathode symmetrical cell),after 150 hrs of operation at 870° C., 14% O₂-3% H₂O, and 300 mA/cm²,including an LNF CCC layer, a nickelate cathode layer, and a GDC10cathode barrier layer in the configuration shown in FIG. 2 after aging.In the example of FIG. 12A, the cathode was formed of a composition withthe formula (Pr_(0.5)Nd_(0.5))₂NiO₄ without any GDC. Conversely, in theexample of FIG. 12B, the cathode was formed of a composition with theformula (Pr_(0.5)Nd_(0.5))₂NiO₄-30% GDC20. As shown, the example cathodeof FIG. 12B displayed fine microstructure and better adhesion to thecathode barrier layer as compared to the example cathode of FIG. 12A.

FIG. 13 is a plot illustrating the results of a short-term durabilitytest of cathode asymmetric button cells (anodic side has the samematerial) with different example cathodes: 1) (Pr_(0.5)Nd_(0.5))₂NiO₄(“PNN5050”); 2) (Pr_(0.5)Nd_(0.5))₂NiO₄-30 wt % GDC10 (“PNN5050-30%GDC10”); and 3) (Pr_(0.5)Nd_(0.5))₂NiO₄-15%(Pr_(0.24)Nd_(0.21))Ce_(0.55)O₂ (“PNN5050-30% PNDC2421”). In this plot,cell ASR is defined as (ASR_(anodic)+ASR_(cathodic)+ASR_(ele))/2. Thesebutton cells included of GDC10 as a cathode barrier layer and LNF as acathode current collector. As shown in the plot, it can be seen that thecell degradation rate was decreased from about 0.3 ohm-cm²/1000 hrs forthe PNN5050 nickelate cathode to about 0.1 ohm-cm²/1000 hrs forPNN5050-30% GDC10, and then about 0.03 ohm-cm²/1000 hrs for PNN5050-30%PNDC2421, which also had lower ASR. As discussed in previous paragraphs,when the cathode composition was changed from (Pr_(0.5)Nd_(0.5))₂NiO₄(“PNN5050”) to (Pr_(0.5)Nd_(0.5))₂NiO₄-ionic composite cathode, theundesired third phase was significantly decreased (compare FIG. 3B andFIG. 4B). When the cathode composition was changed from(Pr_(0.5)Nd_(0.5))₂NiO₄-GDC10 to (Pr_(0.5)Nd_(0.5))₂NiO₄-15%(Pr_(0.24)Nd_(0.21))Ce_(0.55)O₂ composite phase, the phase constitutionwas further improved after aging and the third undesired phase waseliminated.

FIG. 14 is a plot illustrating the results of a short-term durabilitytest of cathode asymmetric button cells with the following examplecathodes: 1) (Pr_(0.5)Nd_(0.5))₂NiO₄ (“PNN5050”); 2)(Pr_(0.48)Nd_(0.52))₄Ni₃O₁₀ (“PN4N3”); and 3) PN4N3-30 wt % (PrNd)O_(y).As shown in FIG. 14, the cell degradation rate was decreased from about0.09 ohm-cm²/1000 hours for the PN4N3 nickelate cathode to about 0.04ohm-cm²/1000 hours for PN4N3-30 wt % (PrNd)O_(y). Both of thesedegradation rates compare favorably to PNN5050 which had a degradationrate of about 0.3 ohm-cm²/1000 hrs. Additionally, PN4N3-30 wt %(PrNd)O_(y) exhibited a lower ASR than PN4N3, and both the PN4N3 and thePN4N3-30 wt % (PrNd)O_(y) cathodes had lower ASRs than PNN5050.

FIG. 15 is a plot of results from a long term durability test ofnickelate composite cathodes ((Pr_(0.5)Nd_(0.5))₄Ni₃O₁₀ (PN₄N₃)-15 wt %GDC10 and PNN5050-15 wt % GDC10) using subscale cell withsegment-in-series cell design (the active cell was printed on poroussubstrate) at about 880 degrees Celsius, wet air, and reformate fuel.After about 2,200 hours of operation, the cell ASR was about 0.2ohm-cm², and degradation rate was about 0.002 ohm-cm²/1000 hr for the(PN4N3)-15 wt % GDC10 cathode and about 0.013 ohm-cm²/1000 hr for thePNN5050-15 wt % GDC10 cathode.

FIG. 16 is a plot of results from a long term durability test ofnickelate composite cathode, PNN5050-30 wt % GDC10, using subscale cellwith segment-in-series cell design (active cells were printed on poroussubstrate) at about 860 degrees Celsius, wet air, and reformate fuel.After about 6,600 hours of operation, the cell ASR was about 0.22ohm-cm², and degradation rate was about 0.007 ohm-cm²/1000 hr.

FIG. 17 is bar chart showing cathode polarization (Rp) of variousnickelate composite cathodes with cathode barrier layer tested under 14%O₂-3% steam and 1 bara using cathode symmetric button cells (for buttoncell, the electrolyte is thicker, about 100 micron vs 10 microns forsegmented-in-series cell design). For some nickelate composite cathodeswith 30 wt % ionic phase, polarization was about 0.02 ohm-cm² or lowerat about 790 to about 870 degrees Celsius.

FIG. 18 is bar chart showing cathode polarization (Rp) of variousnickelate composite cathodes with cathode barrier layer tested under 14%O₂, 3% steam conditions, and 4 bar using cathode symmetric button cells.For some nickelate composite cathodes with 30 wt % second oxide phase,polarization was about 0.014 ohm-cm² or lower at about 790 to about 870degrees Celsius. In addition to the lower ASR, the PN4N3-30 wt % PrNdOycomposition showed lower activation energy than the other examplecathode materials.

Various embodiments of the disclosure have been described. These andother embodiments are within the scope of the following claims.

We claim:
 1. In a fuel cell having a cathode comprising a lanthanidenickelate, a method of inhibiting the formation of a lanthanide oxidephase by forming the cathode from a composition comprising a lanthanidenickelate and a second oxide material which adsorbs an oxide formed fromthe lanthanide.
 2. The method of claim 1 comprising doping an A-site ofthe nickelate with a rare earth metal.
 3. The method of claim 2comprising doping the A-site of the nickelate with a rare earth metaland an alkaline earth metal.
 4. The method of claim 2 comprising dopinga B-site of the nickelate with one or more transition metals.
 5. Themethod of claim 1 comprising doping the B-site of the nickelate with oneor more transition metals.
 6. The method of claim 1 wherein thelanthanide nickelate comprises praseodymium.
 7. The method of claim 1wherein the second oxide material comprises rare earth metals with ageneral formula of A_((1-x))B_(x)O_(y), wherein element A and element Bare different rare earth metals excluding cerium.
 8. The method of claim7 wherein the second oxide material conducts ions.
 9. The method ofclaim 7 wherein one or element A or element B is praseodymium.
 10. Themethod of claim 1 wherein the second oxide material comprises rare earthmetals with a general formula of (C_(w)D_(z))Ce_((1-w-z))O₂, whereinelement C and element D are different rare earth metals excludingcerium.
 11. The method of claim 10, wherein the second oxide materialconducts ions.
 12. The method of claim 10 wherein one of element C orelement D is praseodymium.
 13. The method of claim 10 wherein thecomposition further comprises a third oxide material of rare earthmetals with a general formula of A_((1-x))B_(x)O_(y), wherein element Aand element B are different rare earth metals excluding cerium.
 14. Themethod of claim 13 wherein one of element A or element B ispraseodymium.